1. Field of the Invention
The present invention relates to an Al--Mg--Si aluminum alloy sheet (hereinafter referred to as "Al--Mg--Si sheet") having good press-formability and, in particular, good surface properties. More specifically, the present invention relates to an Al--Mg--Si sheet for forming which is desirable to be used, for example, as a building material for roofs, interior members, curtain walls and the like, and as a material for utensils, electrical parts, optical instruments, vehicles such as automobiles, railcars, and aircraft, general mechanical parts and the like.
2. Related Art
Conventionally, an Al--Mg alloy has been mainly used as an aluminum alloy sheet having good formability. The Al--Mg alloy has, however, drawbacks such that baking hardenability is low, and that stretcher strain marks are liable to be produced when it is subjected to press forming. Therefore, an Al--Mg--Si alloy is attracting notice as an alloy to replace the Al--Mg alloy. The Al--Mg--Si alloy has advantages such that it has good cold formability and high corrosion resistance, and obtains high strength by aging treatment.
However, as described in Japanese Patent Preliminary Publication No. Hei 7-228956 and Japanese Patent Preliminary Publication No. Hei 8-232052, there is a problem that when an Al--Mg--Si alloy sheet is subjected to forming, surface roughness called "ridging marks" is produced on the sheet surface. The ridging marks are stripe-like irregularities which are produced in the direction parallel to the direction of rolling when the sheet is subjected to forming. They are produced conspicuously especially when forming such as stretch forming, ironing, deep drawing or bulging is applied to the sheet in the direction perpendicular to the direction of rolling. A sheet with ridging marks cannot be used for products which particularly require a fine surface such as an interior member, a camera case, an exterior sheet for an automobile and the like, because of its defective appearance.
Japanese Patent Preliminary Publication No. Hei 7-228956 and Japanese Patent Preliminary Publication No. Hei 8-232052 mentioned above intend to prevent production of ridging marks by strictly controlling conditions in each process such as conditions in hot rolling to thereby produce crystal grains which are fine and random in crystal orientation.
However, those prior art documents do not find out the composition of a sheet in which ridging marks are not produced, and cannot sufficiently meet a recent strict demand for quality of a surface.
The Al--Mg--Si aluminum alloy disclosed in Japanese Patent Preliminary Publication No. hei 8-325663 was developed with attention focused on stretchability, while no consideration was paid to the surface properties. Therefore, the alloy cannot sufficiently meet a recent strict demand for quality of a surface.